Since they were put in place a decade or two ago, use of Dallas-area HOV lanes has required having only two people per vehicle. The idea was to encourage carpooling but not make the requirement so onerous that the lanes would sit empty.

In that regard at least, the plan seems to have been successful. The data is somewhat stale, but the five area highways with HOV lanes saw an 8 to 12 percent increase in vehicle occupancy and shaved off as much as a dozen minutes from carpoolers' commute times.

But on at least one major freeway, it may soon take an additional body to be considered HOV-worthy.

"The policy has been up to today, we're in a (air-quality) non-attainment area, we build these facilities ... and get carpoolers in them," said Michael Morris, transportation director for the North Central Texas Council of Governments. "We've been doing that close to 20 years, and they're not full."

So with the completion of LBJ Freeway on the horizon, something of a paradigm shift is coming. The new LBJ will still have free lanes, but it will also have tolled lanes that will presumably be less congested. The Regional Transportation Council has already decided that the drivers of HOVs will get to pay less -- but only if there are three or more occupants.

What would happen when a two-passenger vehicle using an HOV lane on a feeder highway merges onto LBJ? Suddenly, their high-occupancy vehicle isn't high-occupancy enough? Well, Morris said, it's time to look at those other HOV lanes, too.

"We're asking the question, is it time to advance all the HOVs to three-plus and sell additional capacity," he said.

The benefits are two-fold: Selling access to HOV lanes promotes efficiency since it allows more cars to use them. It also raises money. How exactly the tolled HOV lanes would be implemented -- whether with automated tolling like NTTA uses or by selling decals/subscriptions to be monitored through on-the-ground enforcement -- is still an open question.

There will be a period of public comment, but Morris expects the RTC to approve three-plus HOV lanes by Thanksgiving.